The present invention relates generally to a method and apparatus for joining a flexible membrane to a rigid backing. More particularly, the invention relates to the method and apparatus for mechanically effecting a connection between a flexible plastic sheet or tube and a metal backing.
Various applications require the joining of a flexible membrane such as a thin plastic sheet, tube or the like to a more rigid member such as sheet metal, either in the form of flat stock or preformed to a container shape. For example, plastic bladders or bags are often inserted into metal containers with the bladder or bag opening being joined to the container rim. Where the plastic merely is to serve as a protective or decorative overlay, it may be desirable to seal the plastic sheet along its edges to the edge or rim of flat metal stock.
In preferred use, the method of the present invention is used to join the ends of a flexible tubular membrane to rigid, metallic end plates and in particular to end plates which are annular, the ends of the tubular membrane being attached about the opening in each end plate. The article thus formed is utilized as an extender module for the production of flat-ended, stuffed casing food products. Such an article is more particularly described in a copending patent application Ser. No. 430,442.
Several difficulties must be overcome when attempting to join a thin, flexible material such as a plastic membrane or the like to a rigid, metallic backing. For example, if a mechanical connection is to be made by crimping or forming an edge of the backing over an edge of the membrane, it may be necessary to over-bend or over-crimp the metal to compensate for the tendency of the metal to spring back when the crimping force is released. Accordingly, care must be taken to prevent this operation from tearing or cutting the membrane.
Another problem to be solved involves aligning or otherwise maintaining the edge of the flexible membrane properly oriented to the edge of the backing.
In the manufacture of an extender module as described in Ser. No. 430,442, the problems of orientation, positioning and crimping are rendered more acute because the flexible membrane is tubular and because each end of the tubular membrane is to be joined about the perimeter of an opening extending through an annular metal end plate. Moreover, the diameter of the tubular membrane is greater than the diameter of the opening through the metal plate.
In the present invention, the problem of positioning and orienting the flexible membrane sheet is solved by first forming a seat along an edge of the metal backing and then placing the membrane across this seat so that a tail or excess portion of the membrane overhangs the edge of the seat. Then, when the edge of the seat is rolled or crimped to capture the membrane, only this excess material or tail is sheared off along the edge of the seat to form a clean, neat appearing connection.
Properly positioning and orienting the flexible membrane preferably is accomplished by laying the membrane over the end of a movable member so as to provide an excess or tail of membrane material at one side of the member. Movement of the movable member can then carry the membrane to the site of the attachment, plant the membrane in the seat, form an edge of the seat over the membrane, and sever the excess or tail of membrane material.
The problem of cutting or tearing the membrane during crimping is solved by placing a bead of resilient material, such as a cord or monofilament of polyethylene or the like on the membrane. The edge of the metallic member is then crimped or formed over this bead to capture the edge of the membrane to the metallic backing. Due to the resiliency of the bead, a capture is made without the need to tightly crimp the edge of the metallic member thereby greatly reducing the likelihood of cutting or tearing the membrane. Also, during the crimping operation the excess portion of the membrane is sheared off to provide a clean, neat appearing connection.
In a preferred method, the monofilament is placed on the working end of a movable member and then the flexible membrane is draped over the monofilament. In this fashion the movable member can be used to plant the membrane into the seat, place the monofilament line against the membrane and into the seat, crimp or roll the edge of the seat over the monofilament line, and sever the excess membrane along the crimped edge.
In the case of joining a tubular membrane about an opening in a metallic plate, preferably a hollow punch is slidably disposed within a movable sleeve. One end of the sleeve is inserted part way through the tubular membrane and air is evacuated from the other end of the sleeve. This draws an end of the tubular membrane over the end of the sleeve and into the bore of the punch. The sleeve and punch are then advanced towards the metallic plate to plant the membrane into the seat at the site of the joint and to crimp and roll the edge of the plate over the membrane.